Dr. Ahmad Redaa
2024-09-25
This Classification was developed by the International Union of Geological Sciences (IUGS) to standardize igneous rock nomenclature.
The IUGS system requires that we determine the mineral components of a rock and plot the percentages of three of those components on appropriate triangular diagrams to determine the proper name.
If a magma is oversaturated with respect to silica, then a silica mineral such as quartz, cristobalite, tridymite, or coesite should precipitate from the magma and be present in the rock. On the other hand, if a magma is undersaturated with respect to silica, a silica mineral will not precipitate and thus will not be present in the rock. This concept divides rocks into three categories:
These rocks contain minerals that, in general, do not coexist with quartz. Such minerals include:
These minerals indicate a silica-undersaturated rock. When calculating a CIPW norm, silica-undersaturated rocks will contain normative nepheline and/or leucite.
The CIPW Norm (Cross, Iddings, Pirsson, Washington Norm) is a method used to calculate the idealized mineral composition of an igneous rock based on its chemical analysis. It provides a way to standardize and classify rocks by computing a theoretical set of minerals (the norm) that would form from a complete crystallization of the rock’s magma under dry conditions at low pressure.
These rocks can be identified as those that do not contain any of the minerals listed above. When calculated in the CIPW norm, silica-oversaturated rocks will contain normative quartz.
Silica-saturated rocks contain just enough silica that neither quartz nor the silica-undersaturated minerals appear. In the CIPW norm, these rocks typically contain olivine or hypersthene + olivine, but lack quartz, nepheline, and leucite.
Source: https://geologyhub.com
Source: https://www2.tulane.edu
Rocks can be classified as alkaline or subalkaline based on their alkali content relative to silica, using an alkali vs. silica diagram.
Note: Very alkaline rocks, which plot well above the dividing line in the alkali vs. silica diagram, are usually silica undersaturated.